1. Field of the Invention
The invention relates to metal oxide nanostructures formed by natural oxidation at low temperatures.
2. Description of the Related Art
Metal oxides have increasing applications in semiconductors due to their wide bandgap and large excitation binding energy. As a typical wide-bandgap semiconductor oxide with a large excitation binding energy (about 60 meV), zinc oxide is one of the more important functional materials having unique properties of near ultraviolet emission, optical transparency, electrical conductivity and piezoelectricity.
In order to develop technologies to obtain ultraviolet lasing from metal oxide (e.g., zinc oxide) nanorods, it is necessary to develop methodologies to produce one-dimensional (1D) metal oxide nanostructures suitable for applications in electronic and optoelectronic devices. Especially, large-scale and low-cost controllable growth of well-aligned ZnO nanorods on suitable substrates is desired for these cutting-edge applications.
Related art methods of producing nanorods include high-temperature vapor-phase processes that are expensive and energy-intensive. The temperatures required for these processes typically range from 900 to 1100° C. As a result, liquid phase coating of substrates with nanoparticles cannot be accomplished using high-temperature vapor phase processes. However, the complex solution chemistry of these schemes has yielded a complex technology prone to producing irreproducible results.
A typical related art approach to producing ZnO nanoparticles entails the synthesis of metal oxides from organometallic precursors using a two-step approach: (i) the formation of metal nanoparticles from an organometallic precursor, and (ii) oxidation of the metallic nanoparticles. Alternately, unstable organometallic systems (which oxidize readily) are utilized. For example, a complicated system of bisalkyl zinc is oxidized in a solution of hexadecylamine and tetrahydrofuran (M. Monge et al. Angew. Chem. Int. ed., 2003, 42, 5321-5324). Other approaches to metal oxide nanoparticles were also reported previously.
Copper oxide has also been of considerable interest because it forms the basis of technologically important materials such as high-temperature superconductors and plays practical roles in catalysis, sensing, and solar energy technology. Conventional methods of forming copper oxide nanoparticles include thermal decomposition, oxidation, reduction and hydrolysis of metal or metal salts. However, there is still a need for simple technologies to form copper oxide nanomaterials from solutions at moderate, i.e., room, temperatures from simple aqueous solutions.
Accordingly, modern semiconductor and laser technology requires reliable and reproducible new metal oxide materials that are easy and inexpensive to prepare.